Steam power stations

ABSTRACT

1,098,116. Preheating air for combustion; feed water heating. BABCOCK &amp; WILCOX Ltd. March 1, 1966 [March 1, 1965], No. 9039/66. Headings F4A and F4B. [Also in Division F1] During normal operation of a steam turbine plant, combustion air supplied by a fan 8 to a boiler 1 is preheated in a heater 4 by steam bled from a turbine 3 while part of the feed water from a condenser 13 is preheated in a heater 5 by further steam bled from the turbine, the remainder of the feedwater being preheated in a flue gas cooler 12. At peak load, valves RA, RE are closed and valves RD, RC are opened, and the combustion air is then heated by an auxiliary combustion chamber 6. Since steam is no longer being bled from the turbine 3 to supply the heater 4, the turbine develops extra power to meet the peak load. The valves RA, RE, RD, RC can be set to intermediate positions at loads between normal and peak. At superpeak load a further increase of turbine power is provided by operating a valve 20 to reduce or stop the bleed of steam to the feedwater heater 5, the feedwater being correspondingly diverted to the flue gas cooler 12 and the heat input to the latter being correspondingly augmented by means of an auxiliary combustion chamber 7 fed by a fan 11. The combustion chambers, 6, 7 operate on liquid or gaseous fuel. A duct 9 with a valve RB enables the heater 4 and combustion chamber 6 to be connected in series. When the boiler 1 is being started up, it can be preheated by the combustion chamber 6. In a modification, the flue gas cooler 12 is supplied with feedwater from a point within the heater 5 so that part of the heater 5 is in series with it and the remainder is in parallel with it.

Jan. 9, 1968 P. H. PACAULT STEAM POWER STATIONS Filed Feb. 25, 1966 INVENTOR. Pierre Henri Pacauh ATTORNEY United States Patent 3,362,163 STEAM POWER STATIONS Pierre Henri Pacault, Ville-dAvray, Seine-et-Oise, France,

assignor to Babcock & Wilcox, Limited, London, England, a corporation of Great Britain Filed Feb. 25, 1966, Ser. No. 530,028 Claims priority, application France, Mar. 1, 1965, 7,388, Patent 1,435,041 Claims. (Cl. 60--67) This invention relates to power plants employing vapor cycles andmore particularly to a steam cycle for peaking load operation.

A maximum electric power consumption rate or peak load, dictates the minimum of power production equipment that must be maintained operable in any given electrical generating system. Since the peak load, which normally lasts only a relatively small fraction of the total operating time, generally far exceeds the minimum load demand of the system, a considerable portion of the power production equipment that must be retained under operable conditions on a daily basis is actually not used for a considerable portion of the time. Thus, it is obvious that a large, relatively unproductive capital investment is represented by peaking load capacity, i.e. the amount of generating capacity in excess of that required for average power production.

Several methods, including gas turbines, pumped bydrosystems, and specially designed low cost peaking load boilers especially designed for a relatively low efiiciency, short duration operation, have been resorted to in an effort to minimize the capital investment represented by peaking load capacity requirements. Depending on the particular conditions prevailing in a power system, one or several of these methods may be useful in reducing the peaking load capacity capital investment; however, even these methods entail a considerable investment, and furthermore, the efficient integration of these peaking load methods into a standard thermal power generation station may cause certain operational difiiculties.

Obtaining peaking power by firing a boiler of the normal type at a rate in excess of its nominal full load rating is also not an acceptable method, because the materials used in fabricating the superheater and reheater heat exchangers are normally already being utilized at their design limits during normal full load operation.

It is therefore an object of the present invention to provide a steam power plant cycle having incorporated therein peaking load capacity without the attendant limita-- tions which characterize present peaking operation. Further objects of the invention are that a minimum of capital investment be represented by this peaking load capacity, and that operational difliculties be minimized in that the peaking load capacity is attainable within a steam cycle employed for normal full load power generation.

These objects are attained in a steam cycle which includes a steam generator connected for delivery of steam to a 'multi-stage turbine, at some intermediate stage of which steam is normally withdrawn or extracted for purposes 'of heating a fluid .(e.g., feedwater and/or combustijon air) for subsequent delivery to the vapor generator. According to the invention, means are provided for reducing or completely discontinuing the withdrawal of steam from. the-turbine so that the steam normally withdrawn is. free to further expand through the turbine to thereby increase the total turbine power output. During such periods ofpeaking load output, an auxiliary heat source provides for heating of the fluid normally heated by steam extracted from the turbine. Preferably the auxiliaiy heat source includes one or more separately fired combustion chambers which generate hot combustion chamberswhich generate hot combustion gases to 3,362,163 Patented Jan. 9, 1968 be used in lieu of extracted or bleed steam for auxiliary fluid heating purposes.

Where extracted turbine steam is normally used to heat feedwater, the hot combustion gases from the separately fired combustion chamber may be substituted to heat the feedwater in an indirect heat exchanger such as a gas cooler. Where extracted steam is normally utilized to preheat combustion air, as for example in a steam-coil air heater, the combustion air may instead be heated by the products of a separately fired combustion chamber. The ultimate in simplicity and low cost may be obtained in such a separately fired air heater by employing the direct firing method, wherein the incoming cold combustion air is used as a combustion supporting medium in the separately fired combustion chamber, and the gaseous combustion products are passed along with the heated combustion air to the steam generator.

For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the following description which refers to the accompanying drawing which is a schematic diagram of a thermal power plant vapor cycle for production of peaking load power according to the present invention.

Referring to the drawing, the depicted steam cycle in which the inventive concept may be employed includes a boiler or steam generator 10 of the usual type wherein fuel and air are reacted to produce high temperature combustion gases from which heat is transferred to a vaporizable fluid to produce highly superheated steam. A steam outlet line 11 connects the boiler 10 with a multi stage turbine 12 through which the steam expands to produce mechanical energy, the exhaust steam being discharged from the turbine 12 into a condenser -13 of the usual type.

The condensate from the condenser 13 is recycled to the boiler 10 vby a feedwater pump 14. During operation of the cycle in its normal load range, feedwater passes from the pump 14 through line 15 to the feedwater heater 16 and thence through line 17 to the economizer 13 which is formed as an integral part of the boiler 10. In the feedwater heater 16, :feedwater is heated, either directly or indirectly, by steam bled or extracted from a number of turbine stages, which bleed steam passes to the heater 16 via a plurality of lines 12A which are fitted with valves 12B. During normal operation, a portion of the teedwater may be heated during its passage through a flue-gas cooler 20 through which feed'water may be passed in parallel flow relation with the feedwater heater via inlet and outlet lines 19 and 21 with lines 15 and 17 associated with feedwater heater 16. The flue-gas cooler 20 is arranged for the indirect heat exchange between feedwater flowing therethrough and gaseous combustion products from the boiler 10, which combustion products enter the cooler 20 via duct 22 leading from the outlet of the economizer 18 and leave via duct '23 for eventual discharge into the atmosphere. Valves 15A, 17A and 19A are provided respectively in lines-15, 17 and 19, and it should be appreciated that by manipulation of these valves, the feedwater can be passed through the feedwaterheater 16 only, the gas cooler 20 only, or in any proportions through both in par allel flow relationship. For example, during normal full load operation it may be desirable to pass a major portion of the feedwater through the heater 16, and the remainder through the cooler 20, the flow through the latter being determined by the amount of heat available in the flue gases leaving the economizer 18.

Combustion air is normally supplied to the boiler 10 by the forced draft fan 30 via duct 31, air heater 32 and duct 33, which ducts are respectively provided with dampers 31A and 33A. The air passing through the air heater 32 is heated in indirect heat transfer relationship by steam extracted from the several stages of the turbine 12, which steam passes from the turbine 12 to the air heater 32 via a plurality of lines 12C which are fitted with valves 12D. By-pass duct 34, provided with a damper 34A, is arranged in parallel flow relationship with the air heater 32. It should be recognized that with this arrangement, the flow of air from the fan 30 can be regulated so that all or any portion thereof may be passed through the air heater 32.

When the load demands on the cycle exceed the normal full load rating, peaking power may be obtained by decreasing or completely eliminating the withdrawal of steam from the intermediate stages of the turbine 12, so that the normally extracted steam further expands through the turbine 12, thereby increasing turbine power output. It should be noted that with the arrangement shown, the use of bleed steam in the feedwater heater 16 and/or the air heater 32 may be controllably reduced or eliminated.

When the quantity of steam extracted through lines 12A is reduced, it is necessary to supplement the heat input to the feedwater in order that the feedwater supplied to the boiler be maintained at the desired temperature. This supplemental heat is obtained, according to the invention, from a separately fired combustion chamber 40 to which air is supplied through ducts 41 from a fan 42, and fuel is supplied through line 43. Hot gaseous combustion products from the combustion chamber 40 are passed via duct 44 through the gas cooler 20 (along with flue gas from the boiler 10) in indirect heat exchange relationship with the feedwater passing therethrough. The rate of firing in the combustion chamber 40 and the rate of feedwater flow through the gas cooler 20 are regulated to heat the feedwater to the same temperaturre at which it is supplied to the boiler 10 during normal operation. If steam withdrawals through lines 12A for feedwater heating are completely eliminated to obtain maximum peaking power, the feedwater heater 16 may be removed from service by closing valves A and 17A, in which event total feedwater supply would be passed through and heated in the flue-gas cooler 20.

To supplement the heat input to the combustion air when the quantity of steam withdrawn from the turbine through lines 12C is reduced, a second combustion chamber 50 is provided in parallel with the air heater 32. Air is supplied to the combustion chamber 50 from the fan 30 through branch duct 51, and fuel is supplied through line 52. To simplify the design and minimize the cost of equipment required for heating air during peaking load operation, combustion chamber 50 is preferably arranged as a direct fired air heater so that the discharge therefrom consists of heated air slightly diluted with inert gaseous combustion products, the whole of which may be passed directly to the combustion chamber of the boiler 10 via duct 53. The duct 51 is provided with a damper 51A so that the air flow to combustion chamber 50 may be shut off during normal operation. It should be recognized that the flow of air in parallel flow relation through the air heater 32, the by-pass duct 34 and the combustion chamber 50 can be apportioned in any desired manner by manipulation of dampers 31A, 34A and 51A. Moreover, damper 55 is provided in the duct 51 (upstream of the point of connection of by-pass duct 34) so that it is possible to pass combustion air serially through the air heater 32 and combustion chamber 50 via by-pass duct 34. To accomplish this, dampers 55 and 33A are closed and dampers 31A, 34A and 51A are opened. The serial passage of air through the air heater 32 and combustion chamber 50 may also be advantageous during normal or peak load operation to increase the air preheat temperature for purposes of offsetting the adverse affects of abnormally high moisture content of the fuel being fired in boiler 10.

It should also be noted that the combustion chamber 50 could also serve to preheat combustion air for the boiler 10 during start up. This would eliminate the problem (normally experienced during a cold start up) of maintaining combustion conditions when cold air is supplied to the fuel preparation and burning equipment.

In view of their relatively small percentage of operating time, the combustion chambers 40 and 50 may be designed and constructed relatively inexpensively without the normal regard for efliciency. Moreover, it is preferred that liquid or gaseous fuel be fired in these chambers since these fuels require a minimum of equipment for fuel preparation and ash handling.

It is contemplated that power output from the turbine 12 may be increased by up to 15% above normal full load rating by eliminating turbine steam withdrawals to the feedwater heater 16, and that an additional 15% of power output can be obtained by eliminating turbine steam withdrawals to the air heater 32. From the foregoing it will be appreciated that, by use of the present invention, peaking power can be obtained within these limits to any extent desired, and that the capital equipment investment to obtain this peaking power will be lower than has heretofore been possible with known systems. Moreover, the operating characteristic of the boiler 10 and safety of operation will not be jeopardized by the peaking operating procedures disclosed herein.

What is claimed is:

1. A peaking load steam cycle including a vapor generator, fluid heating means associated with said vapor generator, a multi-stage vapor turbine connected to said vapor generator for delivery of vapor therefrom, means connected between the stages of said turbine for normally withdrawing vapor therefrom, means for passing the withdrawn vapor through said fluid heating means in heat exchange relationship with the fluid flowing therethrough, means for reducing the withdrawal of vapor from said turbine during peaking load operation, whereby the vapor normally withdrawn is free to perform additional work in further expanding through said turbine, and auxiliary heat source means connected with said fluid heating means for supplying heat thereto when the flow of withdrawn vapor is reduced.

2. A steam cycle according to claim 1 wherein the auxiliary heat source means includes a separately fired combustion chamber for generating hot gaseous combustion products.

3. A steam cycle according to claim 1 wherein said fluid heating means includes feedwater heating means connected to said vapor generator for delivery of a heated vaporizable liquid thereto, and said auxiliary heat source means is connected with said feedwater heating means for supplying heat thereto when the flow of wi thdrawn vapor to said feedwater heating means is reduced.

4. A steam cycle according to claim 3 wherein the auxiliary heat source includes a separately fired combustion chamber for generating hot gaseous combustion products, and said feedwater heating means includes a gas cooler wherein said gaseous combustion products are passed in indirect heat exchange relationship with the vaporizable fluid being heated.

5. A steam cycle according to claim 2 wherein said fluid heating means includes an air heater connected to said vapor generator for delivery of heated combustion air thereto, and further including a fan normally supplying air to said air heater, and means for diverting the flow of air from said fan to said auxiliary heat source for passage in heat exchange relationship with the gaseous combustion products generated in said combustion chamber.

6. A steam cycle according to claim 5 wherein at least a portion of the air from said fan passes directly into said combustion chamber and constitutes a combustion supporting medium therein.

7. A steam cycle according to claim 6 wherein the products of combustion from said combustion chamber pass to the vapor generator along with the heated combustion air.

8. A steam cycle according to claim 3 wherein said fluid heating means further includes an air heater connected to said vapor generator for delivery of heated combustion air thereto, and further including a fan normally supplying air to said air heater, and means for diverting the flow of air from said fan to said auxiliary heat source.

9. A steam cycle according to claim 8 wherein said auxiliary heat source includes a pair of separately fired and operable combustion chambers for generating hot gaseous combustion products, and further including means for passing the air normally supplied to said air heater in heat exchange relationship with the gaseous products of combustion generated in one of said combustion chambers, and a gas cooler connected to the other combustion chamber for indirect heat exchange between said vaporiza'ble fluid and combustion products.

10. A steam cycle according to claim 9 wherein at least a portion of the air from said fan passes directly into the first named combustion chamber and constitutes a combustion supporting medium therein.

References Cited UNITED STATES PATENTS 1,838,007 12/1931 Smith 6095 3,016,712 1/1962 Taylor 60-67 MARTIN P. SCI-IWADRON, Primary Examiner. ROBERT R. BUNEVICH, Examiner. 

1. A PEAKING LOAD STEAM CYCLE INCLUDING A VAPOR GENERATOR, FLUID HEATING MEANS ASSOCIATED WITH SAID VAPOR GENERATOR, A MULTI-STAGE VAPOR TURBINE CONNECTED TO SAID VAPOR GENERATOR FOR DELIVERY OF VAPOR THEREFROM, MEANS CONNECTED BETWEEN THE STAGES OF SAID TURBINE FOR NORMALLY WITHDRAWING VAPOR THEREFROM, MEANS FOR PASSING THE WITHDRAWN VAPOR THROUGH SAID FLUID HEATING MEANS IN HEAT EXCHANGE RELATIONSHIP WITH THE FLUID FLOWING THERETHROUGH, MEANS FOR REDUCING THE WITHDRAWAL OF VAPOR FROM SAID TURBINE DURING PEAKING LOAD OPERATON, WHEREBY THE VAPOR NORMALLY WITHDRAWN IS FREE TO PERFORM ADDITIONAL WORK IN FURTHER EXPANDING THROUGH SAID TURBINE, AND AUXILIARY HEAT SOURCE MEANS CONNECTED WITH SAID FLUID HEATING MEANS FOR SUPPLYING HEAT THERETO WHEN THE FLOW OF WITHDRAWN VAPOR IS REDUCED. 